Electrical conductivity probes are used in a variety of applications to determine liquid levels in vessels and also to actuate fuel cutoff interlocks and alarms automatically, to energize solenoid valves, and to control pump motors at preset levels. Electrical conductivity probes are most frequently used to monitor liquid levels in boilers. Other conventional steam generating applications include installations on feed-water heaters, flash tanks, deaerators and turbine drains. The electrical conductivity probes must be able to withstand pressures up to about 3000 psi and temperatures up to about 700.degree. F.
Electrical conductivity probes are used with liquids which have at least a slight degree of electrical conductivity, commonly found in most inorganic fluids, such as in steam boilers. Generally, a series of electrical conductivity probes are used along the wall of a pressure vessel or in an adjacent gauge tube. As the liquid in the vessel or gauge tube rises or falls, the fluid contacts one or more of the electrode connector tips in a series of probes. Where the electrical conductor of a probe is in contact only with a gas such as steam, a high resistance is encountered. When the electrical conductor of a probe is in contact with the conductive liquid, such as water, a relatively lower resistance is encountered. When the liquid is in contact with the electrode conductor of a probe and the wall of the pressure vessel or gauge tube, an electrical circuit is completed which, in turn, operates an alarm or some indicating device.
An essential feature of such electrical conductivity probes is an insulator which insulates a metal mounting body and the metal electrode connector. Zirconium oxide is currently used in most electrical conductivity probes of this type since it is one of the few materials that has been found effective for use in high pressure boilers. Aluminum oxide and glass have been found to be unsatisfactory at high temperatures and pressure.
A mechanical pressure type joint is necessary between the insulator and the metal mounting body and the electrode connector. The last three mentioned body parts of the electrical conductivity probe are held in compression by a central rod which is in tension. A feature of the mechanical sealing means between the foregoing body parts of the electrical conductivity probe is that at higher pressures the seals must be constructed to develop increasing sealing pressure.
In order to effect this sealing arrangement, certain electrical conductivity probes presently being marketed are made pursuant to U.S. Pat. No. 3,158,682. The sealing means employed therein is two relatively soft nickel gaskets which are brazed, respectively, to the metal mounting body and the electrode connector. As disclosed in the patent, a conical surface on the nickel gasket must be machined to correspond to another conical surface on the hollow insulator. Slightly different angles are used for the two conical surfaces so that a circumferential line contact is made between the insulator and the two soft nickel gaskets. Alternatively, in actual practice a rectangular gasket can be brazed and then deformed into a conical surface by pressure contact from the hollow insulator.
As can be appreciated from the foregoing description, which is disclosed in more detail in U.S. Pat. No. 3,158,682, machining and assembling involves several steps. First, machining a conical surface on the zirconium oxide insulator is difficult. Second, relatively soft nickel gaskets must be brazed to the metal body parts. Third, the conical surface must be machined on the soft nickel gaskets. Typically, two different angles are disclosed in the patent for the different nickel and insulating parts, such as 57.degree. and 60.degree.. It will be appreciated that such a slight angular difference in such small parts requires a high degree of accuracy. Even if the nickel gasket is not premachined to a conical surface, care must be taken to insure that the conical surface on the hollow insulator is used to accurately deform the nickel gasket to form a mating surface in the soft nickel gasket. In addition, assembly of the electrical conductivity probe of U.S. Pat. No. 3,158,682 requires special assembly steps. The main probe body parts must be aligned very accurately because of the conical surfaces on the seal structure. A special assembly jig is required to hold the parts from rotating while a torque nut on a center rod is tightened to place the main body parts in compression.
A further problem arises out of the use of the electrical conductivity probe of U.S. Pat. No. 3,158,682. If a leak is developed in the probe, the probe must be disassembled to ascertain the location of the leak. In disassembling the probe, the relatively soft nickel gaskets have a tendency to adhere to the insulator. Because of that adherence and because the nickel gaskets are brazed to the metal body parts, disassembly of the probe tends to result in breakage of the hollow insulator. That breakage compounds the ability to make field repairs on the electrical probe. In addition, field repairs are complicated because reassembly of the probe requires a special jig to prevent rotation of the main body parts.